The present invention relates to a light guide for the back light of a passive display device, or particularly a liquid crystal display. Generally, the display device is classified into an active (luminous) type and a passive (non-luminous) type. The active type display device includes a light emitting diode, a plasma display and an electroluminescent display which give displays by emitting light by themselves. The passive type display device includes a liquid crystal display (hereinafter abbreviated as LCD) and an electrochromitter display device which do not emit light by themselves but modulate natural light or the light from other light sources through the control of light transmittance or reflectance so as to give pattern displays. When it is dark around the device, therefore, the passive type display device gives an obscure display unless provided with a special lighting means.
An LCD which consumes little power has wide applications in portable electronic devices such as electronic desk calculators and watches. In most LCD's whose display mode utilizes the optical anisotropy of liquid crystal, superposition of deflector plates is an essential constituent feature, and about 50% of the illumination light is shielded by the deflector plates. When a color filter is provided in the LCD, the light intensity is further reduced by the color filter. Accordingly, it is indispensable to install lighting means in the device for normal operation in a room. Because of the large restriction on the power supply available for the portable electronic display device, brighter lighting with as small power consumption as possible is the most important object in manufacturing the display device.
If the display device is operated, using the built-in lighting means alone, in an environment as bright as outdoors in the day time, the display will be obscure and illegible. An increase in the luminance of the display would, however, result in the increased power consumption by the lighting means, and spoil the low power consumption characteristic of the LCD.